The goal of this proposed project grant continues to be the study, on the cellular and molecular level, of a number of the major problems associated with bone marrow transplantation. Specifically, three areas are addressed: 1) hematopoiesis and engraftment, 2) reconstitution of the normal immune system, and 3) graft versus host disease. To achieve this goal, we have brought together investigators in the areas of hematopoiesis, immunology, molecular biology, and cell biology. There are six projects in the proposed renewal together with a core section. Core 9001 is the clinical bone marrow transplant section. This core is critical for it provides the reagents, that is peripheral blood, bone marrow and other tissues from transplant patients required to achieve some of the aims in the research proposals. The six projects are as follows: 1) The role of hematopoietic growth factors and their receptors during stem cell development. The major goal of this proposal is to further our understanding of the mechanisms that regulates normal stem cell self- renewal and differentiation. 2) Mixed chimerism as an approach to transplant tolerance. A non- myeloblative conditioning regimen allowing the use of allogeneic bone marrow transplant for the induction of donor specific tolerance across MHC barriers has been developed. In this proposal, the mechanism of tolerance induced in mice by achieving this mixed chimerism will be evaluated. 3) The effects of GVHD, cyclosporine, and FK506 on T cell ontogeny. Murine model systems will be used to study the dysfunction of thymic ontogeny that occurs from GVHD or from the immunosuppressive drugs cyclosporine and FK506. 4) The mechanisms of induction of TNFalpha gene expression via la molecules. Studies are directed at determining whether cross-linking of la molecules on host cells by alloreactive donor T cells is an important inducer of TNFalpha release in acute graft versus host disease. 5) Mechanism of immunosuppression by FK506. The mechanism by which the drug FK506 inhibits the immune response of T cells and mast cells will be investigated. 6) Cytokine dysregulation in GVHD. The role of cytokines in GVHD will be evaluated and it will be determined whether IL-1RA is an effective therapy.